So that we can eat and get laid. Sorry. That was crude and entirely unsuitable for Valentine’s Day. Here’s a more professional sounding answer. Our survival involves eating other animals, which means we are participating in an ever-escalating, inter-species arms race. As we develop features for catching them, they develop features for avoiding us. Some think that our intelligence evolved to fill an ecological niche analogous to a cheetah’s speed or a spider’s ability to spin web – it is a highly-developed hunting tool. That’s the inter-species evolutionary explanation. The intra-species theory is much sexier.